The invention relates to an apparatus for producing sample vapor which is to be transferred into an inductively coupled plasma, containing
(a) a graphite tube for the electrothermal vaporization of a sample to be investigated, PA1 (b) annular contacts between which the graphite tube is held and through which a protective gas flow can be passed lengthwise through the graphite tube and which surround the graphite tube at a spacing to define an annulus therebetween through which a protective gas flow can be passed, PA1 (c) cooling blocks in which the contacts are retained, PA1 (d) sample supplying means for introducing a sample through a radial sample infeed opening into the graphite tube, PA1 (e) closure means for closing the radial sample infeed opening of the graphite tube, and PA1 (f) means for conveying the vaporized sample out of the closed graphite tube to the plasma by means of a carrier gas flow flowing through the longitudinal bore of the graphite tube. PA1 (g) said closure means contains a controllable closure member to be applied to the graphite tube for closing the radial sample infeed opening, and PA1 (h) the closure member is controlled by means of an actuating mechanism for movement between a first, retracted position, in which the closure member is laterally offset from the axis of the sample infeed opening, and a second, advanced position in which the closure member is applied to the graphite tube.
"Plasma torches" using an inductively coupled plasma are known in the analytical technique. An inert gas is inductively excited such that a very hot plasma results. Samples are introduced into this plasma.
The plasma can then be used to excite the sample atoms to emit light. The spectral lines which are characteristic of certain looked-for elements are observed. The concentration of the looked-for elements in the sample can be inferred from the intensity of these spectral lines.
The plasma can also be used, however, to ionize the atoms of a sample and to pass the ionised atoms to a mass spectrometer. The plasma serves in this case as the ion source of the mass spectrometer.
It is known to spray sample liquid into the inductively coupled, hot plasma. During such operation, however, all constituents of the sample liquid including the solvent and possibly interfering components enter the plasma and perhaps the mass spectrometer.
Through the publication by Hall, Pelchat, Boomer and Powell entitled "Relative Merits of Two Methods of Sample Introduction in Inductively Coupled Plasma Mass Spectrometry: Electrothermal Vaporization and Direct Sample Insertion" in the "Journal of Analytical Atomic Spectrometry", Volume 3 (1988), pages 791 to 797, it is known to electrothermally vaporize a sample and convey the thus obtained sample vapor into the plasma by means of an argon carrier gas flow.
In the known arrangement the sample is deposited onto a platform which is held between two contacts or electrodes. The platform is heated to a high temperature by means of electric current passing through the contacts such as to vaporize the sample. The platform and the contacts are conjointly seated in a housing which is closed off by means of a cover made of quartz and shaped like a reverse funnel. The cover merges with a conduit leading to the plasma. Argon is passed into the housing and carries the vaporized sample into the plasma.
A publication by Wei-Lung Shen, Caruso, Fricke and Satzger entitled "Electrothermal Vaporisation Interface for Sample Introduction in Inductively Coupled Plasma Mass Spectrometry38 in the "Journal of Analytical Atomic Spectrometry", volume 5 (1990), pages 451 to 455, likewise describes an arrangement in which a sample is electrothermally vaporized and the sample vapor is passed into an inductively coupled plasma. The plasma serves as the ion source of a mass spectrometer.
In this known arrangement a graphite tub is held between two annular contacts. The contacts axially extend around the graphite tube. An annulus is thus formed between the contacts and the graphite tube. Through bores in the contacts, an outer protective gas flow is passed into the region of the two ends of the graphite tube. An "inner protective gas flow" likewise enters through a bore in one of the contacts in the region of a first end of the graphite tube into the longitudinal bore of the graphite tube. The contacts are retained in cooling blocks.
A conduit leading to the inductively coupled plasma extends from the cooling block in the region of the first end of the graphite tube. An axial conduit is inserted through a window at the opposite second end into the graphite tube. The conduit ends in a w-shaped loop which receives the sample and is placed at the center of the graphite tube. A carrier gas can also be introduced through this window into the longitudinal bore of the graphite tube by means of a carrier gas conduit. The graphite tube does not have a sample infeed opening in this construction.
The sample is supplied by inserting the w-shaped loop and electrothermally vaporized in the graphite tube. By means of a carrier gas flow, the vaporized sample is then conveyed from the second end of the graphite tube to the first end and via the conduit to the plasma.
An article by Crabi, Cavalli, Achilli, Rossi and Omenetto entitled "Use of the HGA 500 Graphite Furnace as a Sampling Unit for IPC Emission Spectroscopy" in "atomic spectroscopy", volume 3 (1982), pages 81 to 86, describes the use of a conventional graphite furnace for atomic absorption spectroscopy as vaporization apparatus for vaporizing a sample, whereby the sample vapor is carried along by a carrier gas into an inductively coupled plasma. The therein emitted characteristic spectral lines are observed.
In the known arrangement, the carrier gas flow is introduced through one of the inlets which are provided for the "inner" protective gas flow in the conventional use of such graphite furnaces. The remaining protective gas inlets are closed. A ring made of boron nitride extends between the cooling blocks in which the contacts are retained. The ring is held in a stainless steel housing. The stainless steel housing and the boron nitride ring have openings through which a sample can be infed into the graphite tube through the sample infeed opening thereof. One of the windows, through which the measuring light beam passes during atomic absorption spectroscopy, is removed and replaced by a connector for a conduit leading to the plasma.
In an apparatus for introducing samples into an inductively coupled plasma source mass spectrometer such as known, for example, from U.S. Pat. No. 4,886,966, a lengthwisely heated graphite tube has a radial sample infeed opening and is arranged between electrodes for electrothermal heating. One end of the graphite tube is connected to a protective gas source; the other end is connected to a plasma torch. The samples which are introduced into the graphite tube, are first dried and thereafter ashed or charred by electrothermal heating; during this stage the radial sample infeed opening remains in an open state for driving off the developed vapors or smoke. Thereafter, the radial sample infeed opening is closed by means of a plug and the graphite tube is electrothermally heated to atomization temperature. The inert gas flow is, then, used for conveying the atom vapor into the plasma torch and the mass spectrometer.